1. Field of the Invention
This invention relates to a fuel vapor control apparatus to prevent diffusion of the fuel vapor generated in the fuel tank.
2. Related Art
Conventionally, vehicles and the like have been using a fuel vapor control apparatus to prevent the fuel vapor in the fuel tank from diffusing to the atmosphere. The fuel vapor control apparatus has a canister disposed in a passage connecting the fuel tank and the suction pipe of an engine to adsorb the fuel vapor, and also has a purging valve to open or close to occasionally discharge the fuel vapor adsorbed by the canister into the suction pipe through the passage according to the engine condition and thereby to burn together with the fuel-air mixture. Such a fuel vapor control apparatus employs a rubber hose to connect the canister, the suction pipe and the fuel tank, to provide a fuel vapor purging passage. Therefore, if the rubber hose is bent or eroded, it may cause a damage or breakdown, and the fuel vapor or gas may not be discharged into the suction pipe, and, on the contrary, may be diffused into the atmosphere.
In order to prevent the above trouble, there has been proposed, as disclosed in Japanese Patent Publication No. Hei 4-505491, a system having a switching valve disposed in the canister to open or close to the atmosphere, the purging control valve inserted between the canister and the suction pipe, and a pressure sensor disposed in the fuel tank, thereby to detect failure or abnormality in the fuel vapor control apparatus. When the diagnosis of the failure is initiated, the switching valve is controlled to interrupt supply of the air into the canister, the purge control valve is made to open the purging passage between the canister and the suction pipe, and the pressure sensor detects pressure change in the fuel tank.
The pressure sensor of the above system detects pressure change in the fuel tank caused by negative pressure generated in the suction pipe. However, some other system in which positive pressure is applied into the purging passage by some pressure control means and pressure change in the fuel tank from the negative to the positive is detected is also available to diagnose the failure of the fuel vapor control apparatus.
However, the conventional fuel vapor control apparatuses have the following problems.
That is, dust and other foreign particles contained in the air introduced by the switching valve may break into the switching valve and cause damages to the sealing of the switching valve, thereby to result in leakage in the valve. In other words, dust or the like adhere to the valve member and the seal of the valve is subject to wear as the valve switching operation is repeated. As a result, even when the switching valve is closed, a small amount of the fuel gas may leak out of the valve.
Further, if such leakage takes place in the switching valve, it is difficult to detect failure of the fuel vapor control apparatus even if the pressure in the fuel tank is checked while the purge control valve is opened. In other words, even when the switching valve is being closed, the pressure drop in the purging passage and the fuel tank caused by the negative pressure of the suction pipe does not come up to a value for the sensor to detect. As a result, the failure may not be checked.
The same result is expected in case of a system in which a positive pressure is introduced to the purging passage as stated above.
In order to prevent such foreign particles from breaking into the switching valve, there is proposed, as shown in FIG. 2, a system in which an air intake port of the canister is disposed at the downstream of the air cleaner filter of an engine. However, piping 32 connecting the air intake port 81 of the canister 10 and the air intake port 82 of an engine becomes long and massive, and additional check valves 83 and 84 are required to discharge air under an excessive pressure. As a result, the fuel vapor control apparatus 80 shown in FIG. 2 requires much greater space and is much more difficult to be installed into the vehicle.